INT-3325 Add Redis Channel Message Group Store

JIRA: https://jira.spring.io/browse/INT-3325
JIRA: https://jira.spring.io/browse/INT-1870

Optimized MGS for QueueChannel - uses a LIST for
each channel and LPUSH, RPOP.

* Also fix MutableMessage to be Serializable

INT-1870 Priority Redis Channel Message Store

Supports priorities 0-9 (+ no priority).

Priorities out of that range are treated as no priority.

Polishing - Add Marker Interfaces

* Emit a `WARN` log if a channel is used with a regular MessageGroupStore
* Allow message-store on namespace when defining a priority channel

INT-3325 Polishing; PR Comments

Fix some typos in JavaDocs and Docs